1. Field of the Invention
This invention relates to a method of forming an optical element wherein a glass preform for optical use is softened by heating and subjected to press-forming.
2. Description of the Related Art
Recently, methods of producing an optical element having a highly precise optical functional surface have been developed in which postprocessing, such as grinding, polishing and the like, becomes unnecessary by accommodating a material for forming an optical element, for example, a glass blank preliminarily formed to a certain shape and surface accuracy, within a forming mold having a predetermined surface accuracy and performing press-forming while heating the material.
In such a press forming method, in general, upper forming molds and lower forming molds are slidably disposed within a mold guide member in a position facing each other. A preform is introduced within cavities formed by the upper and lower forming molds and the mold guide member. The forming molds are heated to a formable temperature, that is, a temperature where the viscosity of the preform becomes 10.sup.8 -10.sup.12 poises, in a nonoxidizing atmosphere, for example, a nitrogen atmosphere, in order to prevent oxidation of the molds. The molds are then closed and press forming is performed for an appropriate time period to transfer the shapes of the surfaces of the molds to the surfaces of the preform. Subsequently, the molds are cooled to temperatures sufficiently lower than the glass transition temperature of the preform, the press-forming pressure is removed, and formed optical elements are taken out by opening the molds.
The preform may be preliminarily heated to an appropriate temperature before being introduced into the molds, or the preform may be introduced after being heated to a formable temperature. Alternatively, both the preform and the molds are heated, subjected to press forming and cooled at respective predetermined places while conveying them, whereby it is possible to perform continuous operations and high-speed forming.
Press-forming methods and apparatuses for optical elements which adopt the above-described approaches are disclosed, for example, in Japanese Patent Application Public Disclosure (Kokai) Nos. 48-84134 (1973) and 49-97009 (1974), British Patent Specification No. 378199, Japanese Patent Public Disclosure (Kokai) Nos. 63-11529 (1988), 59-150728 (1984) and 61-26528 (1986).
The assignee of the present application has proposed that, in producing optical elements having a relatively large diameter, in a cooling process after preliminarily heating a preform up to a formable temperature, introducing the preform within molds, and performing a press-forming process, using a temperature schedule for stably releasing formed products while maintaining the surface accuracy of the molds, and a second heating means is provided.
However, the following technical problems which must be overcome are present in press forming processing:
(1) In preliminarily heating a glass preform, appropriate heating means and temperature control means are needed in the process of conveying the preform and time is needed for setting conditions for the heating and temperature control means. If the glass preform is introduced within the mold after preliminarily heating the preform to a formable temperature corresponding to a glass viscosity of 10.sup.9 -10.sup.10 poises, the preform may be deformed or the properties of the preform may change at contact surfaces with a conveying jig during transportation, since the preform is already in a fluid state. If a method, in which a glass preform preliminarily heated to a relatively low temperature is introduced within the molds, heated up to a temperature corresponding to a glass viscosity of 10.sup.9 -10.sup.10 poises, and subjected to press forming, is adopted in order to overcome the above-described problems, volatile constituents will evaporate from the preform and contaminate the surfaces of the molds, since the time to keep the preform within the molds becomes long and the temperature is high. Hence, the life of the molds will be shortened in spite of a continuosly formable forming process.
(2) The volume of a formed glass product shrinks in a cooling process after press forming. If the molds cannot follow the shrinkage at that time, discontinuous surfaces are produced on the surfaces of the formed product, adversely influencing, for example, optical properties of the formed product in forming an optical element, such as a lens or the like. If the molds are opened too early, the surfaces of the formed product peeled from the formed surfaces of the molds will be deformed, since temperature drop is insufficient. As a result, the expected optical functional surfaces cannot be provided. Accordingly, it is necessary to optimize the functioning pressure and the functioning temperature range of a second pressing means which follows the above-described volume shrinkage, and the temperature schedule of cooling so as to shorten the forming cycle.
(3) In forming an optical element, optical surfaces of a formed product are in optical contact with the corresponding forming surfaces (mirror surfaces) of forming molds when forming has been completed. The state of contact is substantially the same for the upper mold and the lower mold. Accordingly, when the upper mold is raised in order to take out the formed product after the completion of press forming, a so-called upper-mold-adherence phenomenon wherein the formed product adheres to the forming surface of the upper mold while contacting the upper mold will in some cases occur. If the upper-mold-adherence phenomenon occurs, it becomes impossible to perform a mechanical unloading operation in which the formed product is taken out by sucking it with a sucking finger. Accordingly, in a conventional forming method, if the upper-mold-adherence phenomenon has occurred, the automatic operation is stopped, and the temperature of the upper and lower molds is further decreased. After the operator has confirmed that the formed product dropped from the upper mold onto the lower mold, the process is manually returned to the original process. Hence, if the upper-mold-adherence phenomenon occurs, the rate of operation as the forming apparatus is greatly reduced. Another problem when the upper-mold-adherence phenomenon occurs is that, if the above-described monitoring by the operator or a sensor for detecting adherence of the formed product to the upper mold is absent, the sucking finger will collide with the formed product while the formed product adheres to the upper mold. As a result, the formed glass product will be cracked or blown out of the mold. If the formed product does not favorably drop from the upper mold onto the central position of the lower mold, the operation of sucking the formed product with the sucking finger in the unloading stage becomes difficult.